Two Probe Hysteresis Freezer Control

Describe your system and processes, and post your config file.
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elburro
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Two Probe Hysteresis Freezer Control

Post by elburro » Wed Jun 20, 2018 2:29 pm

Hi.

I have an old STC-1000 flashed to STC-1000+ firmware and it supports two probe freezer control, where one probe is in a TW in the fermenter and the other is reading the freezer temp. It functions like a kind of cascade control where it would run the freezer to the hysteresis/setpoint values unless a second hysteresis value was exceeded - this would prevent overshoots in temperature or freezeup of wort adjacent to the fermenter wall.

Is there an elegant way to do this in the BCS? I have seen where the second hysteresis value would would be an exit condition that sends you back to the cooling delay state and presumably would keep you there until the freezer temp was above its minimum and could continue.

Has anybody done this? Does it work well? Would you set the exit like a hysteresis (like 20F below setpoint) or simply just at a minimum (say 32F) to prevent freezing?

How does it work for ramping mode? I assume only the control setpoint is actually modified?

Best Regards

elburro

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oakbarn
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Re: Two Probe Hysteresis Freezer Control

Post by oakbarn » Thu Jun 21, 2018 12:53 pm

Why use two probes to control? You could use a probe in the freezer set about 7 degrees below the desired fermentation temp. After five days set the freezer at the fermentation temp. I have found this to work. I did have a second probe in the fermentor to see how it was doing, but it was read only and did not control.

elburro
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Re: Two Probe Hysteresis Freezer Control

Post by elburro » Thu Jun 21, 2018 1:16 pm

That would work, but then its really manually controlling the ferm temp by changing the freezer SP. Hoping to do it automatically, like how a Brewpi/Craftbeerpi/STC100+ can do. I think this will work as described above, just didn't know if anybody had tried it.

My freezer is a vertical with an integral fan so its not as gentle as a normal chest freezer; i need the overshoot/freeze protection.

Thanks man!

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oakbarn
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Re: Two Probe Hysteresis Freezer Control

Post by oakbarn » Fri Jun 22, 2018 6:27 pm

I have found that if I control the Cooling Vessel with a Probe in it at about 7 degrees below the desisred Setpoint it works very well for my setup. I have a 27 gal Fermentor. If yours is smaller it may take less of a Delta. I have found that after 4 days, I need to set the Setpoint at the desired fermentation Temp. I have not had much luck in controlling something with a Temp Probe that is not directly in the vessel. It overshoots too much as you have described. You could use two probes and have the exit set up differently if the freezer gets too cold. You could create and endless loop with a final exit based on a Timer.

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oakbarn
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Re: Two Probe Hysteresis Freezer Control

Post by oakbarn » Wed Jul 18, 2018 8:22 am

I had some old aubrins PID and they had this method as well. You would have to use two outs with the BCS to accomplish that. That being said, I have had very good results with simply controlling the fridge or room temp depending on the placement of the fermentor, We have a cold room that I can get down to the high 50s to low 60's and use a freezer for lower temps.

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